1. Field of the Invention
The present invention relates to an optical module used for optical communications, for example, an optical module such as an optical transmitter, an optical receiver, an optical demultiplexer, or an optical multiplexer. In particular, it relates to the improvement of a package structure and a portion where optical fibers are fixed.
2. Description of the Related Art
In a prior-art optical module, optical coupling between an optical fiber and an optical device such as a photo diode (PD) and a laser diode (LD) was often achieved via a free space by using a lens. Such a lens is used for focusing light. When the light exits once in the free space, it is effective to provide the lens. Also, since the coupling is made via a free space, problems such as the difference in thermal expansion coefficient between the optical fiber and element, stress, and distortion do not occur. A prior-art example will be described with reference to the optical transmitter and the receiver.
FIG. 1 is a longitudinal section showing an example of the transmitter which is most commonly adopted for practical use. An outer fence comprises a metallic circular stem 1, a cylindrical metallic lens holder 2, and a metallic conic ferrule holder 3. The housing has a shape with an axial-symmetric three-dimensional expanse. A pole 4 is provided on the upper surface of the stem 1 and on the side thereof, an LD 5 is fixed. The LD has its end faces directed in the vertical direction so that light is emitted in the axial direction. An upper surface-illuminated type monitor PD 6 is fixed on the central part of the stem 1. A cylindrical cap 7 is attached so that the LD 5 and the PD 6 are surrounded. Furthermore, a lens holder 2 is fixed so that the cap is surrounded. A lens 8 is positioned directly above the LD 5.
A tubular ferrule 9 grasps the tip end of an optical fiber 10. The ferrule 9 is inserted in a hole on the top portion of the ferrule holder 3. The axis of the optical fiber 10 exists on the center line of a metallic package (the stem 1, lens holder 2, and ferrule holder 3). That is, the center of the stem 1, the PD 6, the LD 5, the lens 8, and the optical fiber 10 are lined on the center line. The lens holder 2 and the ferrule holder 3 each having a three-dimensional structure are aligned with respect to the x-y surface and the ferrule 9 is aligned in the z direction, and they are then fixed. Herein, lead pins 11 are terminal ends to connect to an external electric circuit.
This is an example of an LD module, however, a PD module having a similar three-dimensional structure is also used. Herein, illustration thereof is omitted. In either three-dimensional structure, a beam of light is perpendicular to the stem surface and the optical fiber protrudes from the top portion of the package. The ray of light exits from the optical fiber to the free space and enters the optical element, or it exits from the optical element and enters the optical fiber via the free space.
The module having such a three-dimensional structure covered by the metal package is advantageous in that it can shut out external noise, has a long life without being affected by moisture and oxygen in the open air, and also has high reliability.
The present structures of the LD module shown in FIG. 1 and the PD module are excellent. However, there is a considerable number of components, alignment thereof takes much time and labor, and the production cost is high. In addition, they have the drawback of having a large shape. The shape and structure without change has limited cost reduction.
Therefore, in order to achieve further reduction in cost and size, active research and development has been devoted to modules of a surface-mount type, etc. A structure of a module wherein an optical fiber is fixed parallel to a bench surface so that a beam of light is parallel with a substrate surface (package surface) is generally referred to as the surface-mount type. In the surface-mount type, various types are included. In some of the types, a free space (air, vacuum) is provided between an optical fiber and an optical device, and in other types, the space between an optical fiber and an optical device is covered by a resin. The present invention relates to a module wherein the section between the optical fiber and devices is covered by a resin. The object of the present invention is to provide a module that the section between the optical fiber and devices is covered by a resin. Now, a prior art wherein such a section is covered by a resin will be described.
For example, a structure as in FIG. 2 has been studied. A horizontal Si substrate 12 is mounted on a horizontal package terrace 13. A Planar Lightwave Circuit (PLC) layer 14 is formed on the Si substrate 12. This comprises an SiO2 layer formed thereon by oxidizing the Si substrate or an SiO2 layer by sputtering. In fact, by doping Ge, etc. on a part of the SiO2 layer, a part with a high refractive index is linearly formed and it serves as a light guide (waveguide). On the Si substrate 12, an LD 15 is fixed on an extension of the waveguide (axis). And immediately behind thereof, a PD 16 for monitoring is mounted. This serves to monitor LD power and to maintain the LD power stably.
A fixing portion 17 is provided at the front end of the waveguide, and in a hole 18 thereof, the tip end of an optical fiber 19 is inserted and fixed. The optical fiber 19, the waveguide, the LD 15, and the PD 16 are lined up straight on the substrate surface. Light which is emitted from the LD propagates in parallel with the surface of the substrate. Therefore, the structure is referred to as the surface-mount type. A transparent silicone resin 21 covers the terminal end 20 of the waveguide, the LD 15, and the PD 16. The light which is emitted forward from the LD 15 propagates through the transparent resin 21 and enters the waveguide terminal end 20. The light which is emitted rearward from LD 15 propagates through the transparent resin 21 and enters the PD 16. The light from the LD propagates only through the resin without going out to the free space. Naturally, the resin must be transparent since the light passes therethrough.
However, since the transparent resin 21 lacks moisture resistance and stress resistance, the outside thereof is covered by a black epoxy resin 22. Since the epoxy resin becomes a hard and solid coating when being hardened, the epoxy resin is excellent in airtightness, mechanical strength, and moisture resistance, etc. Thus, by double-sealing the PD and LD by means of two types of resins with different properties, necessary characteristics such as moisture resistance, stress resistance, and strength are realized, while allowing light to pass therethrough.
The prior-art example of FIG. 2 has been suggested, for example, in {circle around (1)} xe2x80x9cHighly reliable resin-sealed LD-PD modulexe2x80x9d by Fumio Ichikawa, Mitsuo Fukuda, Yasufumi Yamada, Kuniharu Kato, Koji Sato, and Hiroshi Toba, in the General Convention of the Electronic Society of 1998, C-3-161, p.327.
A silicone resin which is transparent and flexible is used for portions through which light needs to pass such as the PD, LD, and the end of the waveguide. The outside thereof is covered by a strong epoxy resin, whereby environment resistance is enhanced. There are a fewer number of components. Because of the sealing means of the resins, the module is lower in price than a metal package. Because of the surface-mount type, the time and labor for alignment is unnecessary. Since the structure does not have a three-dimensional structure but has a two dimensional structure, a smaller size can be achieved.
While such an element as described above has been newly suggested, it has not yet reached the stage for practical use. If a simple module structure becomes possible, a small and low-price optical module can be provided, so that optical communication may spread widely to ordinary homes. This suggestion is promising.
This structure is characterized by double-sealing by means of resins. An inside transmittance (transparent resin) and an outside opaque resin are complementarily utilized. A transparent resin is used on the inside in order to suppress the reflection of light on the boundary between the waveguide and space. For example, a silicone-based resin is used. The transparent resin not only transmits light but also reduces reflection on the end face, whereby the light is suppressed from spreading.
There are not only such optical advantages but also mechanical advantages. The inside transparent resin does not harden into a solid but is in a gel form and is flexible, that is, a soft resin. Since the resin covers the LD and PD, it also serves to protect these semiconductor chips from the outside area. The transparent resin has flexibility and also has an effect to protect a wire for conduction. If the wire is covered over by a hard resin, the wire can be broken due to shocks, etc., however, since the wire is covered by the flexible resin, it is prevented from being cut.
Since the transparent resin itself is soft, it does not cause damage or distortion to the chips and the optical fiber. Such a characteristic of softness is advantageous in the mechanical aspect.
However, from the viewpoint of reliability that is required in the module, the transparent silicone resin has drawbacks. It has high permeability and lacks moisture resistance. The fact that it is flexible means, on the other hand, that a fixed shape cannot be obtained.
Therefore, such a resin as this is entirely covered by an epoxy-based resin in order to fix the shape and improve the moisture resistance. The epoxy-based resin is excellent in hardening ability and fixes its shape. It is excellent in shapeability. It is also excellent in moisture resistance and high in reliability. In addition, the epoxy-based resin does not have transmittance and is suitable to prevent light from entering from the outside area. Such a double sealing resin structure as {circle around (1)} has the abovedescribed advantages.
However, in fact, this double sealing structure has some disadvantages. First, a silicone-based resin is coated on the end of a waveguide and narrow portions between the LD and PD, then is hardened by means of heat and ultraviolet ray, etc. It is preferable if the resin forms a round shape due to surface tension and stops in a condition where about only the area of the PD, LD, and the end of the waveguide are covered. However, in fact, it is hardly possible. This resin is high in fluidity and it does not easily stick to and fit to the substrate. Therefore, the resin slips and flows about the horizontal surface of the substrate. There is a case where the silicon-based resin does not stay at a fixed point and flows due to vibration, inclination, the direction where the resin drops, and instability in supplying pressure. There is also a case where it flows excessively, coats widely, and solidifies on the substrate.
Since the resin is used to enhance smoothness, even if an epoxy resin is coated thereon, the epoxy resin is repelled and flows. That is, an excessive amount of silicone-based resin hinders adhesion of the epoxy resin. If the substrate surface is exposed, the epoxy resin can be adhered, however, if the substrate surface is broadly covered by the silicone based resin, the epoxy resin does not fit. Then, since it cannot be closely fitted to the substrate, the resin comes off. In addition, if it broadens to a lead frame, it covers the upper surface of the lead frame. In this case, wire bonding becomes impossible, and wiring cannot be performed.
Such excessive fluidity, detachability, and lubricity of the silicone-based resin cause a problem. This is the most serious drawback. In addition, there are other weak points. There is also a possibility that stress is applied on the silicone resin as the base resin when it is covered by epoxy and the wire is broken. Also, a gap may be produced in the optical path due to the pressure by the epoxy resin. When a black epoxy resin enters the gap, optical loss increases remarkably. There may be a case where a part of the optical element is exposed by being pushed by the epoxy resin.
The device of FIG. 2 has the resin-double covering structure, however, since there is nothing to restrict the silicone-based resin as the base to flow, the abovementioned disadvantages exist.
A first object of the present invention is to provide a package structure wherein fluidity of a silicone resin as the base is restricted so that a silicone-based resin does not widely cover a substrate. A second object of the present invention is to provide a package structure wherein a silicone-based resin is prevented from flowing into an unnecessary part, whereby a strong covering of an epoxy resin becomes possible.
A package structure of a module of the present invention comprises an inner barrier structure to store a soft silicone-based resin in a gel form and an outer shell structure composed of a fixing resin which surrounds the outside of the inner barrier structure. That is, an inner container for a silicone-based resin is provided inside the package. Therefore, a container having an inside-and-outside double structure is provided comprising the inner container having the barrier inside thereof and the outer container. The inner container and outer container can be formed of the same package material. Otherwise, the outer fence portion on the outside may be formed not of the package material but by hardening an epoxy resin. An optical fiber end and optical devices such as an LD, and a PD, etc. are provided in the inner container and they are filled with a silicone-based resin. Thereon, an epoxy resin is poured to fill the outer container of the package and solidified. The package has the double structure as well as the resins do.
A transparent silicone-based resin is filled inside the inner container. This base resin completely covers the optical devices and optical fiber end. It also covers the wire. However, since the inner container is surrounded by walls, the resin having fluidity does not overflow. The resin does not flow freely on the substrate. The resin does not moisten broadly on the substrate to hinder the epoxy resin from fitting. The effect of the barriers of the inner container is the most advantageous in terms of holding the transparent resin (inner resin) without being spilled.
As shown in and after FIG. 3, the inner resin (transparent resin, silicone-based) 35 is transparent and guides light to the section between optical fibers or between an optical fiber and an optical element. The refractive index of the resin is almost the same as that of the optical fiber so that reflection on the end face is minimal and the light does not spread. The inner resin also acts to protect the wire from being applied with a stress.
The outer resin (fixation resin such as an epoxy) 36 hardens, whereby it protects the inner structure. Since the outer resin does not serve to guide light, it can be black. It also has an airtight-sealing function to prevent water from entering by trickling down a lead frame.
The present invention provides a package having a structure of a plane-mount type and an inside-and-outside double structure comprising an inner container having a barrier and a hard outer container, wherein a glue that is high in fluidity can be stored in the inner container. The range where a silicone-based resin spreads can be limited in the production process with accuracy. The inner container is especially novel and it brings about a benefit in the production process. The outer container provides mechanical strength and airtightness. When a silicone-based transparent resin sticks to a package surface, it repels an epoxy resin and reduces the adhesion, however, the silicone-based resin does not flow out in the present invention. Therefore, a fixing resin of the epoxy base, etc. can form a firm outer shell portion at all times.
The present invention provides a double resin structure comprising flexible and rigid resins which is skillfully combined and it has a complementary effect. Since the inside is made of a transparent resin, light transmission between an optical fiber and an optical element are not spoiled. Since the resin is in a gel form, it reduces stress, the stress is not applied on the optical element, and a wire is prevented from being cut. These are the effects of a completed transparent resin product. Furthermore, since a hard resin is used on the outside, the strength for fixing an optical fiber is high. The structure is airtight and has excellent moisture resistance. The circumference of the lead frame can be fixed by the epoxy resin, whereby the strength and moisture resistance can be improved. Also, since the package structure is made of the resins, mass production is possible and the optical devices and optical fiber can easily be mounted at low cost. According to the present invention, reduction in cost and size of a wide range of optical modules can stably be realized.
Thus, the spread of optical transmission to each home is more actively enhanced.